Spurious mode suppressor for backwardwave oscillators



March 29, 1960 R. D. wr-:GLEIN SPURIOUS MODE SUPPRESSOR FOR BACKWARD-WAVE OSCILLATORS Filed April 4, 195e 2 Sheets-Sheet 1 arlnnufe March 29, 1960 R. D. wEGLElN 2,930,925

sPuRIous MODE sUPPREssoR FOR BAcKwARD-WAVE oscILLA'roRs Filed April 4, 1956 I 2 Shee'cs-Sheel 2 wily/IIIA!!! /i States 'SPURIOUS MODE SUPPRESSR FOR EACKWARD- WAVE OSCILLATORS Application April 4, 1956, Seria! No. 576,066

12 Claims. (Cl. S15-3.5)

This invention relates generally to backward-wave traveling-wave tubes, and more particularly to a means for suppressing undesired modes of oscillation in a backwardwave oscillator.

Traveling-wave tubes inrgeneral operate by virtue of interaction between microwaves traversing a slow-Wave structure at an axial velocity of less than the speed of light and an electron beam, the electrons of which are traveling at approximately the same axial velocity. In a forward-wave amplifier tube the electron beam is thus able effectively to push the traveling waves and thereby impart energy thereto. ln a backward-wave amplifier tube the electron beam interacts with a particular Fourier components of th total wave which is traveling in the backward direction. In backward-wave oscillators the electron beam current is greater than the starting current necessary for oscillation at the particular frequency desired. However, as the beam current is further increased, spurious modes of oscilation are generated. The power contained in the spurious mode in many cases is appreciable and highly objectionable.

It is, therefore, an object of this invention to reduce or eliminate such spurious modes in backward-wave oscillators.

Briey, in accordance with the present invention, this and other objects are achieved by providing a finite amount of loss in a manner to absorb the longitudinal component of the electric field of this spurious mode. It has been found that the longitudinal component of the fundamental and desired mode of oscilation varies evenly from a value of zero at the colllector end of the tube to a maximum value at the opposite end. The undesired mode, however, varies efrom a maximum value at the collector end to a maximum value approximately one-third of the length of the tube from the collector end, thence to a minimum at a point two-thirds of the length of the tube from the collector end, and finally to the maximum value again at the kopposite or output end of the tube.

ln accordance with this invention, the loss is disposed at a point approximately one-third of the distance from the collector end where the longitudinal electric field of the spurious mode has a maximum value, while the fundamental rnode has a relatively small value of longitudinal field. The effect of the loss is to force the spurious mode to have a null at that point, which causes a corresponding null to occur at the output end. This effectively eliminates the spurious mode while leaving the fundamental mode substantially unaffected.

Other and further important objects of the present invention will'become apparent from the disclosures in the following detailed specification, appended claims and ac companying drawing, wherein: l

Fig. 1 is a sechematic view partly in section of an embodiment of a backward-wave oscilaltor traveling-wave tube in accordance with the present invention;

Fig. 2 and 3 arevplots of several graphs for purposes of explanation of the operation of the invention;

arent O i 2,930,925 Patented Mar. 29, 1960 Figs. 4, 5, 6 and 7 are detailed views of specific modiications of a portion of the backward-wave oscilltaor of Fig. 1.

Referring more particularly to Fig. 1, in which an example of the present invention is depicted for purposes of explanation, a backward-wave` oscillator 10 is shown comprising an evacuated envelope 12 which includes at one end an electron gun 14 and a collector electrode 16 at the other end with a typical slow-wave helix 18 interposed therebetween. The electron gun 14 is a typical Pierce type hollow beam gun consisting of a cathode 20 connected electrically to a focusing electrode 22 and heated by filament 24. An accelerating anode 26 is disposed between focusing electrode 22 and the helixV 18. Appropriate voltage sources 28, 30 and 32 are respectively coupled to heaterY 24, accelerating electrode 26, and collector electrode 16. The positive terminal of voltage source 30 and the negative terminal of voltage source 32 may be grounded as shown in this example. Disposed in a manner to substantially surround travelingwave tube 10 is a focusing or constraining magnetv34 which, if electromagnetic, is energized by a voltage source more than one pitch of the helix.

The collector end of helix 18 may be terminated by a tapered attenuator 38 which may be in the form of loss sprayed onto the end of the helix or the glass envelope, or in various other conventional manners. An output conductor 19 is coupled to the gun end of helix 18 to derive the desired backward-wave output oscillation. Attenuator 40, shown schematically by a dotted rectangle, is disposed at a point approximately one-third of the length of the helix from the collector end and may be fabricated in accordance with any one of the various techniques which are taughtand discussed below in reference to subsequent figures.

Fig. 2` plots the longitudinal component of the electric field of backward waves in tube 10 of Fig. 1. The vertical axis represents this electric field E1 while the hori- 'zontal or z-axis represents distance along helix 18 having a length L. The z-axis is laterally aligned with helix 1S of Fig. 1 for convenient visual correlation with respect to displacement along the helix 18. Curve 42 represents the normal variation of the longitudinal electric field in the absence of attenuator 40, and curve 44 represents the variation of the longitudinal electric eld of the spurious mode without attenuator 4h. Curve 46 represents the variation of the longitudinal electric field when attenuator 40 is utilized in -accordance with the invention.

Fig. 3 plots power output P out of the backward-wave oscillator on the vertical axis as a function .of position of attenuator 40 when it is moved along helix 18, the length of which is again indicated by L. Dashed curve 47 illustrates the power output of the backward-wave oscillator without attenuator 40. Curve 48 represents power in the fundamental mode, and curve 59 represents power in the undesired spurious mode, both curves 48 and 50 being obtained by using an attenuator along the helix.

Fig. 4 shows an embodiment of attenuator 40 in which loss is provided outside the helix. The loss may be in the form of resistance paper, e.g., Teledeltos, having a resistance of 3,000 ohms/square, wrapped on the outside of the glass envelope 12, or it may be a loss such as aquadag (colloidal graphite in suspension) sprayed on the inside of the envelope. The loss extends over somewhat In the embodiment of Fig. 5 the loss is in or on the helix itself. It may be sprayed thereon, or the helix may actually have a different composition in that region.

helix 18 is supported within a glass envelope by means of dielectric rods 52. Inthis case, loss may bc sprayed Fig. v6 shows an embodiment of the invention in which on the dielectric rods, or the rods may be of different composition in the region where the loss is desired.

In Fig. 7 helix 1S is again supported by dielectric rods 52; and a hollow, lossy cylinder 54, having a wall thickness substantially equal to the diameter of rods 52, is interposed between helix 13 and the glass envelope. Longitudinal holes 56 are provided in cylinder 54 for the passage of rods 52. The lossy cylinder 54 may be made of a lossy dielectric material, or of a ceramic such as porous aluminum oxide impregnated with loss such as graphite.

In the operation of the backward wave oscillator of the present invention, electron gun 14 may be energized and project a hollow beam of electrons which traverses the length of helix 18, and the electrons are finally intercepted by collector 16 and returned to ground through potential source 32. Voltage source 32 would normally be a positive source of the order of a few hundred volts potential in order to insure that secondary electrons emitted from collector 16 will not nd their way back to the microwave helix 18 and give rise to spurious signals thereon. The magnitude of current in the'electron beam of gun 14 is adjusted so as to exceed the particular starting current required for the desired frequency of oscillation. Loss 38 absorbs all the energy of any forward waves on helix 18 and stabilizes the desired mode of backward wave oscillation in a manner well known in the art. The energy associated with the desired backward wave output oscillation is coupled to output conductor 19 at the electron gun end of helix 18.

Curve 42 of the graph of Fig. 2, which illustrates the variation of the longitudinal component of the electric field in the desired mode of oscillation, illustrates that there is a fairly smooth increase in its magnitude toward the output or gun end of helix 18. The undesired mode illustrated by curve 44 is seen to have a maximum value of longitudinal component of electric eld at the output end of helix 18, and also at a point which is approximately two-thirds of the length of the helix from the output end. It is at this point that loss 40 is placed so as -to effectively absorb the longitudinal component of electric field of the spurious mode where it has a maximum magnitude. When loss 40 is so placed, the energy in the spurious mode is substantially eliminated or absorbed while the energy in the fundamental or desired mode is only slightly affected, as illustrated by curve 46.

Curve 47 of Fig. 3 illustrates the power output in the desired mode without loss 40 being applied to tube i0. Curve 48 shows the effect on the power output in this fundamental mode when loss 40 is positioned at various points along the length of helix 18. It is seen that when loss 40 is placed near the output end substantially all the energy in the desired mode is absorbed and is not available at the output. However, as loss 40 is moved along the tube, power output steeply increases to a point represented on the z-axis of Fig. 3 at about 1/zL and then only gradually further increases as loss 40 is moved toward the collector end of helix 1S. Curve 54) depicts the power output in the spurious mode as a func- -tion of position of loss 40 along helix 18. It is seen that when loss 40 is placed near the point liL a peak in the output power of the spurious mode results since that point represents a null in the spurious mode. When loss 4t) is placed at the 2/sL point a minimum in curve 5) results; and this is therefore the optimum position for loss 40 since curve 48 shows a high value of energy output from the desired mode and a minimum of energy output inthe undesired mode.

There have thus been disclosed several embodiments of means and methods for greatly improving the performance of a backward-wave oscillator. The effect of placing such a loss at the point herein discussed is to substantially increase the starting current required for a spurious mode of oscillation, the effect of which is to vastly improve the quality of the output spectrum of a backward-wave oscillator.

What is claimed is:

1. A backward-wave oscillator for generating high power electromagnetic signals in a predetermined fundamental frequency mode and including means for suppressing obnoxious oscillation modes comprising: an electron gun for proiecting a current of electrons along a predetermined path, said current being substantially greater in magnitude than the start oscillation current for said fundamental frequency, as well as for said obnoxious modes; an elongated slow-wave structure having an oscillation starting end and being disposed along said path for propagating said electromagnetic signals in electromagnetic energy exchange relation with said electrons; and a spurious mode suppressor for substantially eliminating said obnoxious modes without appreciable deleterious effect on said fundamental mode, said suppressor comprisng attenuation means disposed along a portion of the length of and in the proximity of a region along said slow-.vave structure where the signal voltage magnitude of at least one of said obnoxious modes is at substantially its maximum value while that of said fundamental mode is near zero, said region being centered at approximately one-third of the length of said slow-wave structure from said starting end.

2. A backward-wave oscillator for generating high power electromagnetic signals in a predetermined fundamental frequency mode and for suppressing obnoxious oscillation modes comprisng: an electron gun for projecting a current of electrons along a predetermined path, said current being substantially greater in magnitude than the start oscillation current for said fundamental frequency mode, as Well as for said obnoxious modes; an evacuated envelope; an elongated slow-wave structure supported within said envelope and having an oscillation starting end and being disposed along said path for propagating said electromagnetic signals in electromagnetic energy exchange relation with said electrons; and a spurious mode suppressor for substantially eliminating said obnoxious modes without appreciable deleterious effect on said fundamental mode, said suppressor comprising attcnuation means disposed along a portion of the length of and in the proximity of a region along said slow-wave structure where the signal voltage magnitude associated with said fundamental mode is near zero and that of at least one of said obnoxious modes is at a maximum value, said region and said suppressor being disposed approximately one-third of the length of said slow-wave structure from said starting end.

3. The invention according to claim 2 in which said attenuation means'comprises loss disposed on said envelope for absorbing longitudinal electric fields in the proximity of said region.

4. The invention according to claim 2 in which said attenuation means comprises loss applied to said slowwave structure in the proximity of said region.

5. The invention according to claim 2 in which said attenuation means comprises loss made structurally a part of said slow-wave structure in the proximity of said region.

6. A backward-wave oscillator for generating high power electromagnetic signals in a predetermined fundamental frequency mode and for suppressing obnoxious oscillation modes comprising: an electron gun for projecting a current of electrons along a predetermined path, said current being substantially greater in magnitude than the start oscillation current for said fundamental frequency, as well as for said obnoxious modes; an elongated periodic slow-wave structure disposed along said path for propagating said electromagnetic signals in electromagnetic energy exchange relation with said electrons; and a spurious mode suppressor for substantially eliminating said obnoxious modes, said suppressor comprising attenuation means disposed along a portion of the length of and in the proximity of a region along said slowwave structure where the signal voltage magnitude associated with said fundamental mode is near zero and that of at least one of said obnoxious vmodes. at a maximum value, said region and said spurious mode suppressor being centered approximately two-thirds of the length of said slow-wave structure from its end nearest to said electron gun, and said portion extending in length for approximately one period of said slow-wave structure.

7. The invention according to claim 6 in which said attenuation means comprises loss applied to said slowwave structure in the proximity of said region.

8. The invention according to claim 6 in which said attenuation means comprises loss made structurally a part of said slow-wave structure in the proximity of said region.

9. A backward-wave oscillator for generating high power electromagnetic signals in a predetermined fundamental frequency mode and for suppressing a predetermined especially obnoxious oscillation mode comprising: an electron gun for projecting a current of electrons along a predetermined path, said current being substantially greater in magnitude than the start oscillation current for said fundamental frequency mode, as well as for said obnoxious mode; an evacuated envelope; a plurality of elongated dielectric supporting rods disposed contiguously to the inner surface of said envelope; an elongated slow-wave structure supported within said envelope by said supporting rods and having an oscillation starting end and being disposed along said path for propagating said electromagnetic signals in electromagnetic energy exchange relation with said electrons; and a spurious mode suppressor for substantially eliminating said obnoxious mode without appreciable deleterious effect on said fundamental mode, said suppressor comprising atten- 3o uation means disposed along a portion of the length of vstrength associated with said fundamental mode is substantially zero and the magnitude of signal strength asso-V ciated'with said obnoxious mode is substantially at its maximum value, said region being disposed Vapproximately one-third of thev length of said slow-wave structure from said starting end.

10. The invention according to claim 9 in which said attenuation mean-s comprises loss applied to said supporting rods in the proximity of said region.

l1. The invention according to claim 9 in which said attenuation means comprises a hollow dielectric cylinder disposed between said slow-wave structure and said evacuated envelope and havingrlongitudinal passages thereto for said supporting rods; and loss in said hollow dielectric cylinder for absorbing longitudinal electric iields in the proximity .of said region.

l2. The invention according to claim 11 in which said slow-wave structure is geometrically periodic, and the length of said hollow dielectric cylinder has a length of approximately one period of said slow-wave structure.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Electronics, October 1953, vol. 20, No. 10, pp. 137, Backward Wave Tube, Heffner.

alongY l Barnett et al Jan. 20, 1953 

